We have explored CTL-target interactions using well characterized cloned human CTL. Conjugate formation between these CTL and many antigen-negative targets is almost as efficient as with specific target cells, but does not lead to target cell lysis. These findings suggest that, on specific target cells, adhesion by antigen-independent pathways may occur concurrently with or precede antigen recognition. Our MAB inhibition studies using individual MAB and mixes: 1) demonstrate that CD18, CD2, and LFA-3 are involved in antigen-independent conjugate formation; and 2) suggest that the CD2 and LFA-3 molecules are involved in one pathway and the CD18 molecule is involved in a distinct pathway. Confirmation of the existence of distinct pathways was provided by findings that CD18-dependent adhesion requires cations and is temperature-sensitive while CD2- and LFA-3-dependent adhesion does not require cations and is temperature-insensitive. Together with previous data, these studies strongly suggest that CD2 on the effector may interact with LFA-3 as its ligand on targets. Purification of the LFA-3 protein is in progress. Cloning of the LFA-3 gene is being attempted by "shotgun" transfection and by oligonucleotide probing of a cDNA.